Phenolic antioxidants structures

One of the present authors (31) has developed a series of additives which combine the features of both free radical inhibitors and flame retardants of the tetrabromophthalimide or chlorendic imide type with hindered phenol antioxidant structures such as the following compounds ... 

Among the plant phenols, the flavonoids and the anthocyanidins, belonging to the 1,3-diphenylpropans, have been studied in most detail, mainly because of their potential health benefits. With more than 4,000 different flavonoids known, systematic studies of the effects of variation in molecular structure on physico-chemical properties of importance for antioxidative effects have also been possible (Jovanovic et al, 1994 Seeram and Nair, 2002). Flavonoids were originally found not to behave as efficiently as the classic phenolic antioxidants like a-tocopherol and synthetic phenolic antioxidants in donating... 

Figures 10 and 11 show the structure of the hindered phenolic antioxidant Irganox 1010 (Ciba) and its negative ion APCI mass spectra, respectively. Separation was achieved under the following LC conditions Column Aqua Cl 8 (Phenomenex) 3 pm, 150x2.00 mm, 15% carbon loading, proprietary end capping. Column Temp 50°C. Injection volume 5 pi.

We have recently evaluated the chlorendic imide/hindered phenol for its effect on the oxygen index of polyethylene, and we found only a miniscule increase, not considred statistically significant, in comparison to the same loading of chlorine as chlorendic anhydride. We believe that if the antioxidant approach to flame retardancy is to be successful, special high temperature antioxidant structures must be designed for this purpose. 

Kurata, Y, Fukushima, S., Hasegawa, R., Hirose. M.. Shibata, M.-A., Shirai, T. Ito, N. (1990) Structure-activity relations in promotion of rat urinary bladder carcinogenesis by phenolic antioxidants. Jpn. J. Cancer Res., 81, 754-759... 

After benzoylation, it was possible to analyze together the food substances of varying chemical structures, such as alcohols, esters of 4-hydroxybenzoic acid, phenolic antioxidants, saccharides, and sugar alcohols. The method allowed the determination of these substances in different matrices by the same analytical procedure, using the same cleanup. The preservatives were separated on an RP-18 column. Acetonitrile-water (50 35) or acetonitrile-water-butylmethyl ether (110 35 40) were used as mobile phases. Detection was UV at 230 nm (71). 

The main purpose of the work reported here was to develop a low-cost, effective, and nonvolatile 2,4,6-trialkylphenol antioxidant. We discuss the synthesis of some new types of phenolic antioxidants, particularly those resulting from a-olefin alkylation of phenols. We also report the effectiveness of these stabilizers in polypropylene and speculate on the effect of structure on their effectiveness. 

We have prepared a number of new phenolic antioxidants by alkylating phenol, p-cresol, 2,4-xylenol, and 2,6-xylenol with a-olefins. All show appreciable antioxidant effectiveness in high temperature accelerated tests. In over-all potency, 2,6-dioctadecyl-p-cresol is the best, followed closely by 2,4,6-trioctadecylphenol. For the tests used in this study, molecular weight was found to be the controlling factor in the relationship of structure to effectiveness. 

Besides the reactions between phenols and peroxidic bodies, other factors can influence the activity of antioxidants—e.g., compatibility with substrate and volatility. The results show that under the conditions used the influence of the antioxidant structure is dramatic. In this connection we note agreement of the general conclusions dealing with the influences of pyrocatechol antioxidant structure on the activity in polypropylene at 180°C. and those influences found in Tetralin (28) at 80 °C. Despite great differences in experimental conditions, the sequences of the activities of pyrocatechol antioxidants I-VI were in agreement. Great similarities were also found within each particular group of antioxidants. 

Sudjaroen,Y., Haubner, R., Wilrtele, C., Hull, W.E., Erben, G., Spiegelhalder, B., Changbumrung, S., Bartsch, H.and Owen, R.W. (2005) Isolation and structure elucidation of phenolic antioxidants from Tamarind (Famarindus indica L.) seeds and pericarp. Food and Chemical Toxicology 43(11), 1673-1682. 

Figure 4. Chemical structures of synthetic phenolic antioxidants commonly used in fats and oils.

Phenolic and polyphenolic compounds are the most active dietary antioxidants (14). The structural variation of phenolic antioxidants directly influences their physical properties, resulting in differences in their antioxidant activity. BHA and BHT are examples of phenols, in which the aromatic ring contains alkyl groups (hindered phenols), which are extremely effective as antioxidants (11). 

A hindered phenol commonly used as an antioxidant is 2,6-di-terf-butyl-4-meth-ylphenol (also known as butylated hydroxy toluene or "BHT"). Structures of BHT and other hindered phenol antioxidants are shown in Figure 8.3. Many of these complex structures have lengthy lUPAC names and are frequently called by trade names assigned by manufacturers, e.g., Irganox 1135 from Ciba (now BASF). 

Figure 8.3 Structures of hindered phenol antioxidants. (Reproduced with permission from J. Fink, A Concise Introduction to Additives for Thermoplastic Polymers, Wiley-Scrivener Publishing, Salem, MA, 2010).

Cheng Z, Ren J, Li Y, et al. Establishment of a quantitative structure-activity relationship model for evaluating and predicting the protective potentials of phenolic antioxidants on lipid peroxidation. ] Pharm Sci 2003 92(3) 475-484. 

A wide range of structurally diverse compounds can activate the ARE. Classes of xenobiotics that can stimulate ARE-driven transcription include large planar compounds such as flavonoids and phenolic antioxidants (1), thiol-containing compounds such as isothiocyanates (22,23) and l,2-dithiole-3-thiones (24), heavy metals (25), and heme complexes (26,27). Table 1 shows classes and examples of xenobiotics that are known to stimulate ARE-driven transcription. 

The presence of antioxidants in polyethers has a negative influence on the colour. If the polyether is not very well purified and has a basic pH, the phenolic antioxidants are oxidised to quinonic chromophoric structures, which negatively affect the colour of polyether polyols. This is the reason why it is preferable to add antioxidants to the slightly acidic purified polyol and not to the alkaline polyether. 

Phenolic antioxidants are well known for being melt processing stabilizers as well as long-term thermal stabilizers. In the chemiluminescence measurements on the polyethylene films under oxygen (Fig. 3.3), the antioxidant effect of the phenols is clear when these induction times are compared with those of the free additive polyethylene film (0.73 h) (Table 3.2). The results showed that the structure of the phenolic moiety will be a cmcial factor influencing the stabilization performance. 

Former data about properties of cyclohexadienones are reviewed in42 43 This chapter deals only with derivatives containing a peroxide function in the molecule, because they have a specific importance for the long-term properties of polymers. Compounds derived from phenolic antioxidants have the structure of 4-alkylperoxy-4-substituted 2,6-di-tert-butyl-2,5-cyclohexadiene-l-ones XXXVI (in Scheme 5) or 2-alkylperoxy-2-substituted 4,6-di-tert-butyl-3,5-cyclohexadiene-l-onesLXXIV, where R1 may be alkyl, substituted alkyl, or the residue of a molecule of multi-nuclear phenolic antioxidant and R is the residue derived from the oxidized substrate. 

Another proof for incorporation of the originally present antioxidant into an oxidized polypropylene was provided by 14C-labelled Ionox 330. The structures XC and LXXXV (R = polypropylene residue) were proposed for the reaction product123. It is, however, more probable that the transformation products of tris-phenolic antioxidant are bonded to the polypropylene skeleton in a simpler way for steric reasons than under formation of a polymer network. 

If the transformation product of a multinuclear phenolic antioxidant contains in addition to the alkylperoxycyclohexadienone structure also the unchanged phenolic nucleus, it can possess weak antioxidative properties at temperatures when the thermolysis of peroxide does not play an important role. For example, LXXXVI weakly stabilizes tetrahydronaphthalene still at 65 °C. However, the stabilizing effect is strongly decreased in comparison to the initial antioxidant116. At 120 °C, the same compound affects the oxidation in the same way as the triscyclohexadienonyl derivative LXXXV. 

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